The presence of multiple conjugate but non-intersecting faults in geothermal reservoirs presents issues related to fault interaction in the presence of complex coupled thermo-hydro-mechanical (THM) processes in influencing the triggering of seismicity. We examine alternate strategies in stimulating such a conjugate-faulted geothermal reservoir analogous to that hosting the 5.5 Pohang earthquake (2017). We evaluate the response of the reservoir to both short-term stimulation (1y) and long-term production (10y), both with and without thermal effects – for a large fault (F1) adjacent to a non-intersecting smaller fault (F2) and in a reverse faulting stress regime. Results suggest that the slip on either fault impacts the stress state on the other fault through stress transfer. Reactivation of the minor fault (F2) transfers stress towards to the upper part of primary fault (F1), inducing instability. The slip of the major fault is delayed by positioning the location of injection away from the junction between the two faults – decreasing the injection depth from 4087.5 m to 3712.5 m delays the time to slip by 2.61 y. Furthermore, thermal stress plays a decisive role in prompting late-stage fault reactivation for long-term fluid circulation where pore pressures have already reached steady state. The pattern of thermal unloading follows the path of fluid transport and heat transfer along the faults. Overall, this study not only advances our understanding of mechanisms of injection-induced fault instability in EGS reservoirs with multiple and closely-interacting faults, but also provides insights into how different injection strategies can delay or mitigate induced seismicity.

中文翻译：

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热应力传递在引发共轭断层油藏断层失稳中的作用


地热储层中存在多个共轭但不相交的断层，在影响地震活动触发的复杂热-水-力耦合（THM）过程中存在与断层相互作用相关的问题。我们研究了刺激此类共轭断层地热储层的替代策略，类似于发生 5.5 级浦项地震（2017 年）的情况。我们评估油藏对短期增产 (1 年) 和长期生产 (10 年) 的响应，无论是否存在热效应 - 对于邻近不相交的较小断层 (F2) 的大断层 (F1)以及在反向断层应力状态下。结果表明，任一断层上的滑动通过应力传递影响另一个断层上的应力状态。小断层（F2）的重新活动将应力转移到主断层（F1）的上部，从而引起不稳定。通过将注入位置远离两个断层之间的交界处，可以延迟主要断层的滑动——将注入深度从 4087.5 m 减少到 3712.5 m，使滑动时间延迟 2.61 y。此外，热应力对于促进后期断层重新活动、孔隙压力已达到稳定状态的长期流体循环具有决定性作用。热卸载模式遵循沿断层的流体输送和传热路径。总体而言，这项研究不仅增进了我们对具有多个且紧密相互作用的断层的 EGS 储层注入诱发断层不稳定机制的理解，而且还提供了关于不同注入策略如何延迟或减轻诱发地震活动的见解。